130 THE CIRCULATION. 



same level in both branches, and in a vertical position the 

 height of its column must be the same in both. If, now, the 

 blood is made to flow from an artery, through the horizontal 

 portion of the tube (which should contain a solution of carbo- 

 nate of potash to prevent coagulation) into the descending 

 branch, it will exert on the mercury a pressure equal to the 

 force by which it is moved in the arteries ; and the mercury 

 will, in consequence, descend in this branch, and ascend in 

 the other. The depth to which it sinks in the one branch, 

 added to the height to which it rises in the other, will give the 

 whole height of the column of mercury which balances the 

 pressure exerted by the blood ; the weight of the blood, which 

 takes the place of the mercury in the descending branch, and 

 which is more than ten times less than the same quantity of 

 quicksilver, being subtracted. Poiseuille thus calculated the 

 force with which the blood moves in an artery, according to 

 the laws of hydrostatics, from the diameter of the artery, and 

 the height of the column of quicksilver ; that is to say, from 

 the weight of a column of mercury, whose base is a circle of 

 the same diameter as the artery, and whose height is equal to 

 the difference in the levels of the mercury in the two branches 

 of the instrument. He found the blood's pressure equal in all 

 the arteries examined ; difference in size, and distance from 

 the heart being unattended by any corresponding difference of 

 force in the circulation. The height of the column of mercury 

 displaced by the blood was the same in all the arteries of the 

 same animal. The correctness of these views having been 

 questioned, Poiseuille has recently repeated his observations, 

 and obtained the same results. 



From the mean result of several observations on horses and 

 dogs, he calculated that the force with which the blood is 

 moved in any large artery, is capable of supporting a column 

 of mercury six inches and one and a half lines in height, or a 

 column of water seven feet one line in height. With these re- 

 sults, the more recent observations of other experimenters 

 closely accord. Poiseuille's experiments having thus shown to 

 him that the force of the blood's motion is the same in the most 

 different arteries, he concluded that, to measure the amount of 

 the blood's pressure in any artery of which the calibre is known, 

 it is necessary merely to multiply the area of a transverse sec- 

 tion of a vessel by the height of the column of mercury which 

 is already known to be supported by the force of the blood in 

 any part of the arterial system. The weight of a column of 

 mercury of the dimensions thus found, will represent the pres- 

 sure exerted by the column of blood. And assuming that the 

 mean of the greatest and least height of the column of mercury 



